In a frying pan or on a griddle, cook the chicken and onions together. Chop them up until they are shredded.
While still in pan or on griddle, cut the mixture in half and spread it out to about the size of the hoagie. Lay two pieces of cheese side-by-side over each chicken and onion mixture. Let cheese melt.
Toast each hoagie then spread mayonnaise, two pieces of tomato and a piece of lettuce on each. Add the chicken, onion and cheese to each roll and enjoy.